I want to briefly explain why I joined Abhijit Banerjee, Peter Diamond, Esther Duflo, Paul Krugman and Joe Stiglitz in signing the letter on the California billionaire tax: ft.com/content/94ec0fee-53ab… In principle, I am not convinced that permanent wealth taxes would be necessary if the tax-transfer system were designed optimally. But the US tax system is very far from optimal, and has been for decades. As I have documented in my research (for example, here: mitsloan.mit.edu/shared/ods/…), labor income is taxed much more heavily than capital income. This asymmetry creates two distinct problems. First, it makes the tax system highly regressive at the top. The very rich, who receive much of their income from capital or can use accounting tricks to reclassify their income as capital income, pay remarkably little in taxes. For example, a business owner who runs their own company should receive a significant part of their income as labor earnings for their work as CEO. Instead, they can take their compensation in stock and borrow against those holdings to finance whatever consumption they desire, minimizing their tax obligations. Even their heirs may avoid paying these taxes. Second, the asymmetry distorts automation decisions: it effectively subsidizes machinery and AI relative to hiring workers (for example, here: brookings.edu/articles/does-…). These distortions have allowed a small number of people to amass vast fortunes without paying their fair share of taxes, and have fueled excessive automation. The resulting inequality is a problem in its own right. It is all the more dangerous today because our institutions have become fragile, allowing the very wealthy to exert growing control over the political process. A temporary wealth tax can therefore be justified on three grounds: (1) it partially reverses the effects of more than two decades of tax avoidance by the very wealthy; (2) it acts as a brake on their growing dominance over the political process; and (3) it may pave the way for more comprehensive tax reform at the federal level. The California billionaire tax is not perfect. For example, a federal tax would lessen risks related to capital flight, and removing the rigid earmarking of the revenues for specific purposes would enable the proceeds to reduce the national debt. Nevertheless, with few other options on the table, I believe the California proposal deserves support.